The present invention relates to the determination of phase transition temperatures and, more particularly, to the determination of cloud, fluidity, freezing, and haze points of hydrocarbon fuels and hydrocarbon/oxygenate fuel blends.
Liquid hydrocarbon fuels are characterized by changes in physical form which are critical to its performance and which occur at certain temperatures. The temperatures at which these changes occur are known as "phase transition temperatures" and are commonly used as specifications for hydrocarbon fuels. In general, these phase transitions are defined by the occurrence of visibly observable changes. For example, the cloud point is the temperature at which wax or other insoluble substances begin to crystallize in or separate from a hydrocarbon fuel which has been chilled. Another example of a phase transition temperature which is important in assessing hydrocarbon fuels is "haze point" or "water tolerance test" for gasolines which contain oxygenated octane boosters, such as methanol and/or methyl tertiary-butyl ether (MTBE), in the presence of trace amounts of water. Such measurements are critical for assessing the optimal blending concentration of such oxygenates in gasoline so as to prevent phase separation in fuel tanks and other fuel systems. The separation of these octane boosters from the gasoline phase can have serious deleterious effects on the fuel tanks in which they are stored and on the fuel systems in which they are used. For example, it has been found that methanol may cause deterioration of certain seals and gasket materials currently used in underground storage tanks.
Techniques for determining phase transition temperatures in liquid hydrocarbon fuels have heretofore generally relied upon visual observation by an operator and the subsequent manual recording of the temperature as read from a thermometer. Methods such as ASTM D-2386 (for determining the freezing point of aviation fuels or fluidity point of heating oils and diesel fuels), ASTM D-2500 (for determining the cloud point of petroleum fuels), and the ASTM recommended procedure for determining water tolerance in gasohol are all based on such methodology. While such techniques are generally convenient and inexpensive, they are time consuming and subject to inconsistent visual observation by one or more operators. In addition, the accuracy of such techniques may be greatly influenced by ambient lighting conditions.
Somewhat more sophisticated means for detecting the cloud point of a liquid are currently available. For example, if the cloud point is due to the appearance of crystals having an anisotropic structure, the cloud point may be determined by illuminating the suitably cooled liquid with polarized light. Polarized light, which is normally extinguished in the absence of anisotropic crystals, increases in intensity as it traverses the liquid, this increase being due to the depolarization of the incident rays of light by the appearance of anisotropic crystals. See for example U.S. Pat. No. 3,457,772-Chassagne et al. The apparatus used according to such technique comprises two cells, one a measuring cell designed to receive a liquid to be examined, the other a reference cell. This technique has the disadvantage of being applicable only to liquids having a phase transition which is associated with the appearance of anisotropic crystals.
Another apparatus which may be used for cloud point determination comprises relatively complex and precisely configured primary and secondary light guides positioned so as to project a beam of light from the primary to the secondary light guide via a sample gap. A sensor is provided for responding to a decrease in the intensity of the light emerging from the secondary light guide. See for example U.S. Pat. No. 3,527,082-Pruvot. In order to allow for fluctuations in the intensity of the illumination, primary and secondary photoconductive cells are required in this method. One further disadvantage of this technique is that it requires the balancing of the light intensities at the two photoconductive cells both before use and from time to time as may be necessary during service.
Attempts have been made to overcome the disadvantages of the apparatus and techniques described above by utilizing laser light in place of incandescent lamps. See for example U.S. Pat. No. 3,807,865-Gordon et al. The technique of Gordon et al. uses a relatively complex series of thermal pulses and the measurement of the resultant physical variable changes to determine spinodals and critical points. The use of laser light in the method of Gordon has not reduced the sensitivity of the measuring apparatus to ambient lighting conditions or provided a more compact apparatus.
It has long been desired in many applications to determine phase transition temperatures using a simple and compact apparatus which is not effected by ambient light. For example, a portable and effective cloud point detector would be extremely useful in an industrial setting requiring infrequent but periodic determinations of phase transition temperatures at different locations. Since the ambient conditions at such locations may vary greatly, it is desired for such an apparatus to be both relatively compact and also insensitive to ambient light. Prior apparatus have addressed the problem of ambient light sensitivity by providing light tight housings. See for example U.S. Pat. No. 4,572,676-Biermans. However, such housings are generally expensive; not conducive to the compact nature required of the portable device; and prone to light leaks, especially in the harsh environment of industrial applications.